1. Field of the Invention
The present invention relates to an intra prediction system using a pixel value of an encoded adjacent pixel as a prediction value in a video encoder and a video decoder which allow intra encoding when each macro block (hereinafter, called MB) in a screen is encoded.
2. Description of the Related Art
In a video encoder which allows intra encoding as a encoding selection when a screen is divided into MBs in encoding to encode each of the MBs, pixel values of an encoded adjacent MB are arranged for prediction values of intra encoding of a process MB in a direction suitable for the feature of an image, thereby obtaining high prediction performance. When identical pixel values are arranged in a fixed direction, however, as the pixels are farther from prediction values, the prediction performance becomes lower. Also, in a pattern causing periodic texture, when a certain pixel value is used as a prediction value, high prediction performance cannot be obtained.
In an image representing shading by illuminating an object with a light source and gradation effect, for example, pixel values near a fixed direction are arranged, but they change at a fixed frequency. Therefore, when identical pixel values are arranged along the direction, as the pixels are farther from pixels used as prediction values, prediction performance becomes lower, resulting in reduction of encoding efficiency.
Japanese Patent Application Laid-Open (JP-A) No. 2006-217158, Yazawa et al., “Prediction in pixel-adaptive screen corresponding to the distance of reference pixel” (hereinafter, “document 1”), Annual Conference of The Institute of Image Information and Television Engineers 2006, 13-2, Aug. 2006 and Matsuo et al. (hereinafter, “document 2”), “Study of intra prediction improving method using peripheral adjacent pixel”, research report by Information Processing Society of Japan, 2007-AVM-56 pp. 37-42, March 2007 (hereinafter, “document 3”) disclose such technique.
In the disclosure of the document 1, in order to improve the performance of intra prediction in a frame in the conventional intraprediction direction, the intraprediction direction applied to each MB belonging to a process frame is controlled based on the feature (pan, tilt, zoom, and blur) of an image.
In the disclosure of the document 2, in order to improve intra prediction performance, when a prediction value of a certain pixel value is generated in intra prediction, prediction values of a plurality of adjacent pixels are used. When a prediction value is generated, prediction values of pixels close thereto are used, thereby solving the problem that farther pixels lower prediction performance.
The document 3 discloses an intra prediction performance improving method according to a method of using pixel values near a process MB as prediction values in intra prediction. In the conventional art, only values of pixels close to a process MB are used as prediction values. In this document 3, values of pixels far from the process MB are also used as prediction values of pixels along a predetermined prediction direction.
The invention described in the document 1 is effective for an image obtaining high prediction performance when identical pixel values are used as prediction values. In an image with a pattern in which pixel values are periodically changed, there are concerns that prediction performance is lowered to reduce encoding efficiency.
The invention described in the document 2 generates a prediction value based on pixel values or prediction values of encoded adjacent or predicted pixels. The prediction value can be represented by computation for the locally decoded result of the pixels adjacent to a process MB. Since texture along the prediction directions is not considered, however, in a pattern in which pixel values have a constant change tendency like gradation effect, as pixels are farther from pixels adjacent to the process MB as a source of the prediction values, prediction performance becomes significantly lower. Similarly, an appropriate prediction value cannot be generated in a pattern having periodic texture.
The invention described in the document 3 is a method of allowing pixel values far from a process MB to be used as prediction values. After the pixel values used as prediction values are determined, identical prediction values are set according to a fixed direction as in the conventional art. In a pattern in which pixel values have a constant change tendency like gradation effect, pixels are farther from pixel values used as prediction values, prediction performance becomes lower. Since the identical prediction values are used according to the fixed direction, a prediction value suitable for periodic texture cannot be generated.